Plastic materials are lightweight, highly tough and easy to be dyed, and so recently have been used for various types of optical members, especially for eyeglass lenses in a large amount. The optical members, especially the eyeglass lenses, are required to have good optical properties, and specifically to be high in both the refractive index and the Abbe number. A high refractive index allows the lenses to be thinner and more lightweight and to have a better appearance. A high Abbe number reduces the chromatic aberration of the lenses, and so alleviates the load on the eyes of a wearer of the eyeglasses to make the eyes less likely to be tired.
However, in general, as the refractive index increases, the Abbe number decreases. Conventionally, it has been studied to improve both the refractive index and the Abbe number. The best technique provided so far by these studies uses an episulfide compound, which achieves a refractive index of 1.7 and an Abbe number of 36 (Japanese Patent No. 3491660).
In addition to the high refractive index and Abbe number, the eyeglass lenses are also required to have physical properties such as high transparency, low degree of yellowing, high heat resistance and high strength, as well as a secondary processability of ease of dyeing. In order to fulfill these requirements in addition to the good optical properties, Japanese Patents Nos. 3465528, 3541707 and 3663861 and Japanese Laid-Open Patent Publication No. 11-318960 propose compositions.
Regarding the reactivity of an episulfide group, the episulfide group is homopolymerizable as a result of ring opening reaction progressing in a chained manner when a catalyst such as an amine compound or the like is used. Because of being homopolymerizable, the episulfide group does not require a large amount of curing agent unlike an epoxy resin and needs only a small amount of catalyst to be cured. However, since the episulfide group is not reactive with a variety of substances, the functional groups usable as a comonomer are limited. For example, the episulfide group is unlikely to react with a general-purpose resin monomer generally used, such as acrylic compounds, methacrylic compounds, (hereinafter, occasionally the acrylic compounds and the methacrylic compounds may be collectively referred to simply as “(meth)acrylic compounds”), allyl compounds and the like, are unlikely to react with an episulfide compound. For this reason, it has not been easy to obtain a resin from a composition formed of any of such compounds and an episulfide compound. Such an unlikeliness to react with a general-purpose resin has been an obstacle for expressing good properties of the episulfide compound.
Accordingly, it has been desired to apply the good properties of the episulfide compound to a general-purpose resin while making use of the homopolymerizability, by which the episulfide compound is curable with a small amount of catalyst.
Meanwhile, materials used for eyeglass lenses in the largest amount which have a refractive index of 1.6 still have an Abbe number of merely about 40, and materials having a refractive index of 1.66 still have an Abbe number of merely about 32. No material having better properties has been put into practice. As a result, the materials having a refractive index of 1.6 or more and less than 1.7 are inferior to the materials having a refractive index of 1.7, both in the refractive index and the Abbe number. The materials having a refractive index of 1.6 or more and less than 1.7 are used in the largest amount but do not have an improved Abbe number. For this reason, a material having a higher Abbe number has been desired.
From the point of view of an optical member, it is desirable that a material is adjustable in the refractive index in addition to be improved both in the refractive index and the Abbe number. The best technique provided so far by the studies made to produce such a material realizes a composition for an optical member containing an inorganic compound having a sulfur atom and/or a selenium atom and an episulfide compound. The refractive index of such a composition is adjustable by changing the composition ratio (Japanese Patent No. 3738817). This technique realizes a material having a refractive index of 1.7 or more and less than 1.8 while having a good Abbe number, the refractive index of which is adjustable.
By contrast, regarding the materials used for the eyeglass lenses in the largest amount which have a refractive index of 1.6 or more and less than 1.7, the refractive index thereof is determined in accordance with the compound used. Specifically, in the case of thiourethane materials formed of polythiol and polyisocyanate, which are mainly used as materials having a refractive index of 1.6 or more and less than 1.7, the molar ratio of an SH group and an NCO group needs to be exactly 1 and so the refractive index is not adjustable.
Accordingly, an optical material having a refractive index of 1.6 or more and less than around 1.7 while keeping the Abbe number high, the refractive index of which is arbitrarily adjustable, has been strongly desired. Specifically, a material having a refractive index of 1.6 or more and less than 1.7 and an Abbe number of 35, the refractive index of which is easily adjustable by changing, for example, the composition ratio, has been strongly desired.
Meanwhile, regarding high refractive index materials having a refractive index of 1.7 or more, the optical characteristics have been progressively improved owing to the episulfide compound. However, for middle refractive index materials having a refractive index of around 1.6, which are used for the eyeglass lenses in the largest amount, the Abbe number is still around 40. No material having better properties has been put into practice. Therefore, a material having a refractive index of around 1.6 and a higher Abbe number is desired. Such a material is also desired to have a higher transparency in order to be usable for an optical member.
Optical members, especially eyeglass lenses, are required to have a low specific gravity and a high refractive index, and also good physical properties such as a high heat resistance and a high strength. A low specific gravity allows the lens to be more lightweight, and a high refractive index allows the lens to be thinner. A high heat resistance and a high strength provide a high secondary processability and are also important from a point of view of safety or the like.
The refractive index of the materials used for eyeglass lenses in the largest amount today is around 1.6. Representative examples of such materials conventionally used include methacrylate compounds containing a bromine atom and thiourethane obtained from polythiol and polyisocyanate. Representative methacrylate compounds containing a bromine atom are described in Monthly Report of Japan Chemical Industry Association, the August 1987 issue, pp. 27-31. These materials have problems of being poor in the strength and the heat resistance, which are important elements of the eyeglass lenses, as well as problems of having a high specific gravity due to the large amount of bromine contained and being easy to be colored. The thiourethane materials are described in Monthly Report of Japan Chemical Industry Association, the February 1994 issue, pp. 8-11, Japanese Patent Publications for Opposition Nos. 4-58489 and 4-15249, and Japanese Laid-Open Patent Publications Nos. 8-271702, 9-110955 and 9-110956. These materials are improved in the strength but do not have a sufficient heat resistance. In addition, these materials also have problems of having a high specific gravity due to the large amount of sulfur atoms contained and generating a bad odor when being cut.
So far, lenses having a low specific gravity have been proposed in Japanese Laid-Open Patent Publications Nos. 5-215903, 5-307102, 5-307103 and 7-292043. However, the materials used for these lenses have a refractive index of merely up to about 1.5 and so are not suitable to reduce the lens thickness, and as a result do not realize a lightweight lens. These materials also have problems of being poor in the strength and the heat resistance. For these reasons, the lenses proposed in these publications are not widely used.
Accordingly, a lens material (an optical member) having a refractive index of around 1.6 and having good properties of a low specific gravity, a high strength and a high heat resistance has been desired.